In a compound archery bow, the force required to move the bow string (i.e. the draw force), varies as a function of the draw length. In a typical compound bow, the draw force is initially fairly high, and as the bow approaches a fully drawn condition, the draw force decreases. This let off in draw weight permits an archer to hold a fully drawn bow in a relatively steady position thereby providing an increase in accuracy. As a result of the let off and the force-draw characteristics of the bow the amount of energy that can be stored is maximized thereby providing for a flatter path of travel of the arrow, higher arrow velocity and an increase in the amount of energy delivered to the target. For these reasons, compound bows are widely used by target shooters and bow hunters.
In a typical compound bow, an arrangement of cams or levers, typically disposed at the limb tips of the bow, and operating through a series of cables, is employed to give a mechanical advantage as the bow is drawn and thereby modify the force draw curve. U.S. Pat. No. 3,486,495 describes a typical compound bow of the prior art, and many variations of this basic design are known. The bow of the U.S. Pat. No. 3,486,495, as well as most compound bows presently employed are what is termed dual cam bows because of the fact that a pair of cam elements are employed to control the force draw characteristics of the bow. While dual cam systems are widely employed, they suffer from problems because of the need to keep the two separate cams synchronized. If the cams are out of alignment they can produce an uneven and erratic bow string motion.
A bow string includes a nock point defined thereupon for engagement with the arrow, typically at a location at which the arrow is perpendicular to the string, when supported by an arrow rest associated with the handle portion of the bow. If the cams are out of alignment, the bow string will feed unevenly and the nock point will follow an erratic path when the bow string is drawn and released. This erratic path of travel can induce unwanted oscillations in the arrow, adversely affecting its accuracy. Also, misalignment of the cams will cause the draw characteristics of the bow, and the power delivered thereby to vary. As a result of these problems, some archers have avoided the use of compound bows despite their significant advantages.
In order to overcome the problems of cam alignment, the art has investigated bow designs in which power is delivered through only one cam. U.S. Pat. Nos. 4,365,611 and 5,368,006 disclose single cam bows in which a lower, eccentric pulley provides the camming action, and an upper, concentric pulley guides the motion of the bow string. It has been found that the performance of prior art, single cam bows is less than optimum because of the fact that the nock point of the bow string does not follow a linear path of travel. While this non-linear path of travel is relatively constant for a particular bow, it still adversely affects accuracy. Therefore, it will be appreciated that there is the need for a compound bow in which the nock point of the bow string maintains a linear path of travel. It is further desirable that the bow be capable of storing as large an amount of energy as possible, while providing a significant let off in draw weight at the fully drawn position. The present invention provides an improved single cam bow which incorporates a unique pulley design and in which the nock point of the bow string regularly, and repeatedly, travels along a linear path. These and other advantages of the present invention will be readily apparent from the drawings, discussion and description which follow.